In general, a member forming a rotary part which is provided in a laser printer, a facsimile, a bar code reader or the like must be capable of withstanding high speed rotation. As the printing speed, for example, a rotator of a polygon mirror in a laser printer must rotate at a speed of at least 20000 r.p.m. In a sliding portion of such a rotator, a ball bearing is generally used as a bearing, for example. So far as such a conventional ball bearing is employed, however, the rotator can merely withstand a rotating speed of about 16000 r.p.m. at the maximum, due to problems of burning, wear and the like.
Japanese Patent Laying-Open No. 2-173610 (1990) proposes an air bearing (dynamic pressure gas bearing) of ceramics such as SiC or Si.sub.3 N.sub.4, which is employed for a rotary sliding portion in order to drive a polygon mirror of a laser printer at a high speed. Upon rotation of a rotator which is supported by such an air bearing, air is forcibly introduced at least into a clearance between a radial bearing member or a thrust bearing member and the rotator through a groove. Thus, the air pressure in the clearance is so increased that the rotator can rotate at a high speed with the aid of the air bearing. In order to implement such high speed rotation, rotational accuracy is maintained during high speed rotation through the air bearing, which is also adapted to support a thrust-directional load being applied to the rotator. The radial bearing member and the rotator, which are formed of ceramics, can withstand a sliding between the members in a low speed region which starting or stopping of the rotator.
When a conventional radial bearing member of ceramics such as Si.sub.3 N.sub.4 or the like is employed at a high rotating speed exceeding 5000 r.p.m., however, an impacting knock wear phenomenon results from contact between the bearing members. Specifically, the radial impact force applied to the rotator during high speed rotation causes a sliding between the members in a high speed region, and it is difficult for a conventional ceramic sintered body to withstand such a high speed sliding When the radial or thrust bearing member is formed as an air bearing member which is made of a ceramic sintered body, therefore, it is difficult to support a radial impact force suddenly applied to the rotator during high speed rotation, although the rotational accuracy is maintained and a thrust-directional load applied to the rotator is supported.
When the air bearing member is made of ceramics, further, such a ceramic member must be worked or assembled with a high accuracy, and hence the manufacturing cost is increased. In addition, the air bearing member must be used in a clean environment, to be protected against dust. Thus, the structure related to the air bearing member is so complicated that a closed container may be required to contain the same, and the space therefor is disadvantageously increased. In order to solve such a problem, a bushing type slide bearing member made of ceramics may be used as a bearing for high speed rotation.
FIG. 16 is a longitudinal sectional view schematically showing the structure of a conventional bushing type slide bearing member of ceramics. The known bushing type slide bearing member comprises two thrust slide bearing members 81 and 82, a radial slide bearing member 83 and a rotator 84, which are paired with each other. Such a bushing type slide bearing member can withstand a sliding between the members in a low speed region when starting or stopping. Similarly to the aforementioned air bearing member of ceramics, however, an impacting knock wear phenomenon is caused by the contact between the bearing members as a result of a sliding contact following high speed rotation at a speed exceeding 5000 r.p.m. Therefore, it is difficult for the conventional bushing type slide bearing member to withstand such sliding, and hence any frictional resistance is increased by roughened sliding surfaces. This tendency is particularly remarkable with respect to a thrustdirectional load. A thrustdirectional frictional resistance caused by such a thrust load is 5 to 10 times larger than a radial frictional resistance. Thus, when a bushing type slide bearing member of ceramics is employed as a bearing for high speed rotation, it is difficult to support not only a sudden radial impact force during rotation but also a thrust-directional load which is applied to the rotor of the bearing.